Many hot runner injection molding systems use a valve bushing having a valve pin coupled to a piston that is actuated between closed and open positions when pressurized air is supplied to a piston chamber to control the flow of molding material into a mold cavity. In some known hot runner designs, for example, those used in the molding of PET preforms, the valve bushing is mounted to the hot runner manifold, and an air seal between an actuator portion of the valve bushing and a sealing surface on an adjacent mold plate through which the pressurized air is delivered is required to efficiently actuate the valve pin. In addition, to prevent molding material, and gasses created during processing of the molding material, from escaping the channels of the hot runner system, a close tolerance fit is required between many of the hot runner components. For example, a fluid seal is required between the valve pin and the valve bushing.
Failure to create an air seal between the actuator portion and the sealing surface of the adjacent mold plate will result in leakage of air to the interior of the injection molding system, which may reduce the overall efficiency of the system due to a need to continuously replace leaking compressed air, and can also result in an inability to translate the pistons, and the valve pins connected thereto, between the open and closed positions.
Since the valve bushing is mounted to the manifold, heat input to the manifold not only causes the manifold to expand, but also causes the valve bushing to expand, which may adversely affect the fit between the valve pin and the valve pin bushing thus allowing molding material and gasses created during processing of the molding material to escape the channels of the hot runner system and migrate into the piston chamber.
Heat loss from the manifold into the actuator portion of the valve bushing may also lead to additional problems, for example, premature wear of piston seals, and decreased thermal efficiency of the manifold as a result of the additional heat input to the manifold that is required due to the aforementioned heat loss.
Heretofore solutions to solve many of the problems associated with manifold mounted valve bushings have not proven to be effective, and as such, a need still exists in the art for a valve bushing that reduces or overcomes at least some of the above described deficiencies.